The invention relates to a tomosynthesis apparatus for the formation of layer images of a body. The apparatus has a large number of radiation source positions which are situated in one radiation source plane, a diaphragm device for forming radiation beams which irradiate a common superposition zone, and a detector which is situated in a detector plane behind the superposition zone.
An apparatus of this kind is known from United Kingdom Patent Application No. 2,035,769 (corresponding to German Offenlegungsschrift No. 2847011). The radiation beams are formed by means of diaphragm apertures so that the beams, having substantially equally large beam cross-sections, register in a predetermined superposition plane. As the distance from the superposition plane increases, the common superposition zone of the radiation beams becomes tapered. The radiation beams themselves are recorded as a superposition image on the detector surface in a more or less superposed form.
The formation of layer images of a body, which images are situated within the superposition zone, is realized by imaging the superposition image on a light-sensitive layer, for example a frosted glass plate, by means of an imaging matrix. The imaging matrix may be for example, a lens matrix whose imaging elements or lenses are arranged in the matrix plane according to with the flat distribution of the radiation sources. By displacement of the light-sensitive layer or the imaging matrix in the direction of the optical axis with respect to the detector surface, different layers of the superposition zone within the body can be imaged on the light-sensitive layer. The body layer within the superposition zone is then optimally reconstructed, while the images of the body layers situated above or below the reconstructed layer have a reduced image quality due to the decreasing degree of superposition of the radiation beams.
It is often desirable to image cylindrical body zones whose cylinder axes extend substantially perpendicularly to the radiation source plane by means of such a continuous slice imaging process (that is to say by means of a process which images a large number of approximately equally large layers the images being of substantially the same image quality, and which layers are situated behind one another, viewed in the direction of the cylinder axis). However, such a cylindrical zone can be situated only within the superposition zone. In the superposition plane of all radiation beams, the size of the superposition zone is substantially larger than the corresponding dimension of the cylindrical zone.
Therefore, for the layer imaging of a cylindrical body zone in the known apparatus, a substantially large zone of the body is irradiated in addition to the area claimed to be imaged. This has an adverse effect on the radiation load for the body as well as on the quality of the layer images, because a larger amount of scatter radiation is produced, and hence more artefacts appear in the layer images due to the comparatively strong superposition of the radiation beams on the detector surface.